1. Field of the Invention
The present invention relates to a method for performing delay. estimation and carrier synchronization on quadrature phase shift keying (QPSK) or offset QPSK (OQPSK) modulated code division multiple access (CDMA) systems. More particularly, the present invention provides a method for acquisition of a pseudo-noise (PN) synchronization point as well as Doppler shift in a received carrier signal.
2. Description of the Related Art
The motion of a receiver relative to a transmitter, such as a cell phone relative to a cell site, can generate a Doppler frequency and phase shift in a carrier signal. Such a frequency or phase shift can cause an increase in the bit error rate for synchronized users.
CDMA technology focuses on artificially increasing the bandwidth of a signal as a method of spread spectrum. Bandwidth is increased by breaking each bit into a number of sub-bits called"chips". Assuming each bit is broken into 10 chips, the result is an increase in data rate by 10. By increasing the data rate by 10, bandwidth is also increased by 10.
Each original CDMA bit is divided into chips by multiplying the bit by a pseudo-noise (PN) code. A PN-code is an arbitrary sequence typically ranging between -1 and 1. Multiplying each of the original modulated signal bits by the PN-code results in the original bits being dividing into smaller chips, hence, increasing bandwidth. The-greater number of chips which the PN-code creates results in a wider bandwidth proportional to the number of chips.
To create a transmitted signal with PN-coding, a transmitter first modulates a message at a higher carrier frequency. For spread spectrum, all messages are modulated on the same carrier. After modulation, each signal is then multiplied by a PN-code in the transmitter to spread the bandwidth. Multiplying by the PN-code to spread signal bandwidth can also be done before carrier modulation.
In the receiver, the incoming signal is the spread spectrum signal. In order for the receiver to extract a transmitted message, the incoming signal is multiplied by the same PN-code used in the transmitter. With the PN-code ranging between -1 and 1, multiplying by the same PN-code in the receiver effectively cancels out the PN-code on the particular message.
With a Doppler shift due to the receiver moving relative to the transmitter, cancellation of the Doppler shift must occur to establish synchronization. Synchronization can be divided into two steps: acquisition and tracking. Acquisition is the process of roughly aligning the PN-code of the transmitted signal with the identical PN-code of the received signal. Tracking occurs after acquisition, and maintains a tight alignment of the two PN-codes over time. Misalignment of the transmit and receive PN-codes results in noise being generated in the received signal. The more severe the misalignment, the greater the bit error rate can be for the received signal.
The acquisition process for synchronization in the past required prior clock synchronization, including both frequency and phase synchronization, between the transmitter and receiver, or a prior knowledge of the transmitted data sequence.